1. Field of the Invention
The present invention relates in general to a pressing method and a pressing system, and more particularly to techniques for reducing a shock or impact upon collision of a die with a blank on a pressure ring, by using shock absorbers.
2. Discussion of the Related Art
There is widely used a pressing system including (a) a die and a punch cooperating to perform a drawing operation on a blank to draw the blank along a forming surface of the punch, (b) a cushion platen, (c) resistance applying means for applying a resistance to a movement of the cushion platen, (d) a pressure ring cooperating with the die to hold the blank at a peripheral portion thereof during the drawing operation, and (e) a plurality of cushion pins interposed in parallel with each other between the cushion platen and the pressure ring, for transmitting a blank holding force based on the above-indicated resistance to the pressure ring, wherein the drawing operation is performed when the pressure ring and the die are moved relative to the punch in a pressing direction against the above-indicated resistance. During the drawing operation, the cushion platen is lowered against the resistance applied thereto by the resistance applying means, while the cushion platen has a substantially horizontal attitude. The punch is fixedly positioned on a bolster disposed above the cushion platen. The cushion pins are supported at their lower ends by the cushion platen such that the cushion pins extend through respective through-holes formed through the bolster and respective through-holes formed through the punch. The cushion pins support at their upper ends the pressure ring. The die disposed above the punch and pressure ring is reciprocated in the vertical direction by suitable drive means, so that the die and the pressure ring are moved relative to the punch so as to perform the drawing operation on the blank.
Also known is a pressing system further including (f) a plurality of fluid-actuated balancing cylinders which are disposed on the cushion platen such that the lower ends of the cushion pins are associated with the pistons of the respective fluid-actuated balancing cylinders- The fluid-actuated balancing cylinders are hydraulic cylinders having respective pressure chambers communicating with each other, and the pistons of these cylinders are held in their neutral positions during the drawing operation with the blank held by and between the pressure ring and the die, so that the blank holding force acts evenly or uniformly on the pressure ring through all of the cushion pins. An example of this type of pressing system is disclosed in JP-A-6-304800 (published in 1994). In this pressing system, the blank holding force is evenly distributed to the pressure ring and the blank through the fluid in the hydraulic cylinders and the cushion pins, so as to establish a desired distribution of the blank holding force depending upon the arrangement of the cushion pins, irrespective of dimensional and positional errors or variations such as a length variation of the cushion pins and an inclination of the cushion platen with respect to the horizontal plane.
The drawing operation on the blank is initiated with a collision of the die with the blank and the pressure ring. This collision may cause a considerable noise and/or an oscillatory change of the blank holding force, which may lead to defective products formed by the drawing operation. In view of this drawback, there has been proposed to use a shock absorber device for reducing the shock or impact upon collision of the die with the blank and the pressure ring. An example of the shock absorber device is disclosed in JP-U-60-89933 (published in 1985), wherein shock absorbers are interposed between the cushion platen and the resistance applying means in the form of a pneumatic cylinder for applying a resistance to movement of the cushion platen.
In the conventional pressing system provided with the shock absorber device described above, however, the reaction forces of the shock absorbers act on local portions of the cushion platen, and may cause deflection or bending deformation of the cushion platen, resulting in a risk of uneven distribution of the blank holding force. Even in the presence of the shock absorbers, the blank holding force still undesirably oscillates or fluctuates in an initial period of the drawing operation immediately after the collision of the die with the blank and the pressure ring, due to a reaction force produced by the collision, which reaction force causes operating instability of the various mechanical components of the pressing system such as unstable elimination of backlash of a gear train in the drive means for reciprocating the die. Thus, the provision of the shock absorbers in the conventional pressing system is not effective enough to overcome the drawback.